Chromosomal evolution in the Ridge-tailed goannas

  • Jason Dobry

    Student thesis: Doctoral Thesis

    Abstract

    Chromosomes were the first genetic molecules used for evolutionary studies and are a complex arrangement of proteins and DNA sequences that organise into stable structures for the faithful replication of genetic material. These structures are conserved in all domains of life. Chromosomal evolution often occurs in tandem with species evolution, and the diversity of form and number of chromosomes is highly variable across taxa. The relationship between chromosome evolution and species divergence is a topic of ongoing research with a rich history dating back over a century. Despite this, several questions remain unanswered about the fundamental mechanisms that drive chromosome evolution and the direct impacts that these changes have on the sequence of events that lead to species adaptation, diversification and finally speciation. This thesis addresses fundamental questions regarding the molecular processes that occur during species evolution, and what role chromosome evolution plays during these events. The chromosomal, genic, and geographic processes are typically considered independent variables, but often there are events that occur simultaneously or with considerable overlap, and integrated studies merging technologies and theory are needed for the interpretation of contradictions revealed by one-dimensional studies. In this thesis, I explore the role of chromosome polymorphisms in the divergence of Ridge-tailed goannas, (Varanus acanthurus BOULENGER), a species complex of dwarf monitor lizards from the Subgenus Odatria. This unique group of monitor lizards have a broad distribution across northern Australia and widespread chromosome polymorphisms across a significant portion of their range. However, some populations have fixed karyotypes typical of all other Varanus species in which clusters of species (clades) are defined by their karyotype identity, and the clades differ by chromosome morphology characterised by pericentric inversions on chromosomes 5-8. It is unknown if the chromosome polymorphisms represent a genetic barrier between individuals with different karyotypes, represent a hybrid population, or what genes are involved in these rearrangements. To test for the role these polymorphisms play in species divergence, I used a multidisciplinary, chromosomics approach integrating field ecology, population genetics, genomics, and cytogenetics. Specifically, I tested for geneflow between individuals with chromosome rearrangements within and between populations to address the question: are chromosome polymorphisms a by-product or a driver of population divergence? I proceed to characterise the rearrangements identified on both autosomes and the sex chromosomes. This thesis shows that polymorphisms occurring within chromosomes are central to driving genetic divergence and are actively occurring within populations, and these transitions have occurred de novo in different populations multiple times resulting in a rapid diversification of species in the Varanidae lineage in Australia. I further apply the logic that de novo chromosome rearrangements are drivers of divergence to the broader understanding of vertebrate chromosome evolution and highlight the methodology and its application for addressing these fundamental questions in other vertebrates. Similar structural transitions characterise the diversification of the primates and other vertebrate lineages that have undergone recent and rapid speciation.
    Date of Award2023
    Original languageEnglish
    SupervisorMD Tariq Ezaz (Supervisor), Janine Deakin (Supervisor) & Erik Wapstra (Supervisor)

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